The turbine section of a gas turbine engine includes one or more arrays of stator vanes. Each vane includes a cambered airfoil having a convex suction wall and a concave pressure wall. The walls extend chordwisely from an airfoil leading edge to an airfoil trailing edge. When the vanes are installed in an engine, their airfoils span across a working medium fluid flowpath that carries a stream of hot, high pressure combustion gases discharged from the engine combustion chamber. Each vane also includes one or more internal coolant passageways, bounded at least in part by one of the airfoil walls. One or more spanwisely extending arrays of film cooling holes penetrate the airfoil walls to establish fluid communication between the passageways and the flowpath. A coolant, usually pressurized air extracted from an engine compressor, is introduced into the coolant passageways to cool the airfoils and protect them from the intense heat of the combustion gases. The coolant then discharges into the working medium flowpath by way of the film holes. The discharged coolant forms a protective cooling film over the exposed suction and pressure surfaces of the airfoil walls to inhibit heat transfer from the combustion gases to the airfoil.
The pressure of the coolant introduced into any internal coolant passageway must satisfy conflicting requirements. The coolant pressure must be high enough, relative to the flowpath (combustion gas) pressure, to ensure a positive outflow of coolant through the film holes and into the flowpath. Otherwise the hot combustion gases will backflow through the film holes and enter the coolant passageway. However, excessively high coolant pressure can lead to blowoff, a phenomenon in which discharged coolant penetrates forcibly into the stream of combustion gases rather than spreading out over the airfoil surface to establish a protective cooling film. Between these two extremes, care must be taken to reliably match the coolant pressure to the spanwise density of the film holes (quantity of film holes per unit length in the spanwise direction) at all engine operating conditions. If the coolant pressure is too modest for a given hole density, the coolant flow rate through the film holes will be inadequate to ensure spanwise continuity of the cooling film. This, in turn, leads to undesirable hot spots on the airfoil surface. However, if the coolant pressure is too high for the existing hole density, the vane will use an excessive amount of coolant. Since the coolant is pressurized by energy absorbed from an engine compressor, the excessive use of coolant degrades engine efficiency and increases engine fuel consumption. Of course, the airfoil designer can specify a hole density high enough to minimize the likelihood of cooling film discontinuities, even when the engine is operating at conditions where the compressor supplies the coolant at a relatively low pressure. However, the dense hole array is likely to use an objectionable quantity of coolant when the engine is operated at other conditions where the compressor supplies relatively high pressure coolant. Conversely, the designer can specify a low density hole array to ensure economical use of coolant at high pressure, but only at the risk of introducing cooling film discontinues at conditions where the compressor supplies lower pressure coolant. Finally, elevated coolant pressure imposes high stresses on the airfoil walls since the stresses are proportional to the difference between coolant pressure and flowpath pressure. These high stresses dictate the use of exotic materials that possess high strength, but are exorbitantly expensive.
In some vanes, the arrangement of coolant passageways and film holes complicates efforts to satisfy the foregoing constraints. One such example is a vane with a coolant passageway and two sets of film holes--one set venting the passageway to a relatively low flowpath pressure adjacent the suction surface of the vane, and the other set venting the passageway to a relatively high flowpath pressure adjacent the pressure surface of the vane. A coolant whose pressure is low enough to prevent blowoff and excessive coolant usage at the suction surface of the vane may not provide adequate backflow margin or spanwise film continuity at the pressure surface. Conversely, a higher coolant pressure may improve backflow margin and film continuity at the pressure surface but may encourage blowoff and excessive coolant usage at the suction side.
One way to contend with disparate flowpath pressures along the suction and pressure surfaces of an airfoil is shown in U.S. Pat. No. 5,365,265. The disclosed turbine blade includes a chordwisely extending septum 48 that bifurcates a leading edge channel 46 into independent pressure side and suction side chambers 46a, 46b. Appropriately sized metering holes 72, 74 meter compressed air into the chambers to ensure sufficient backflow margin across film holes 50, 52, 54 while guarding against blowoff of the film cooling air issuing from holes 52. The disclosed blade also includes a visually similar arrangement of chambers 38a, 38b segregated by another chordwisely extending septum 40. Unfortunately, the septa 40, 48 have the potential to complicate blade manufacture and can make the blade susceptible to thermo-mechanical fatigue since the septa do not expand and contract as much as the external walls 20, 22 in response to changes in the temperature of the combustion gases 34.
What is needed is a coolable turbomachinery airfoil that resists both backflow and blowoff, makes efficient use of coolant, does not jeopardize spanwise continuity of the cooling film, is not overly vulnerable to thermo-mechanical fatigue, and has reduced wall stress so that the use of less exotic, more cost effective materials is a viable option.